1. Field of the Invention
The present invention relates to radio base stations, mobile stations, and communication methods.
2. Description of the Related Art
Currently, mobile communication systems enabling wireless communication between a radio base station and a mobile station are widely used. In such mobile communication systems, the radio base station manages radio resources (frequency resource, time resource, etc.) used for the communication from the radio base station to the mobile station (downlink communication) as well as for the communication from the mobile station to the radio base station (uplink communication). The mobile station is allocated an uplink radio resource by the radio base station and performs uplink data transmission or the like by using the allocated uplink radio resource.
In the mobile communication system, part of the uplink radio resource is often assigned to a random access channel in advance. Even while no resource is allocated, the mobile station can transmit a signal (random access signal) via the random access channel. In order to avoid collision of random access signals from multiple mobile stations, the radio base station designates, with respect to each mobile station, a signal (preamble signal) to be sent in the preamble of the random access channel. The following are conceivable uses of the random access channel (see, e.g., 3rd Generation Partnership Project, “Evolved Universal Terrestrial Radio Access (E-UTRA) and Evolved Universal Terrestrial Radio Access Network (E-UTRAN); Overall description; Stage 2 (Release 8)”, 3GPP TS36.300, 2008-03 V8.4.0).
Suppose that the radio base station needs to transmit downlink data to a mobile station, by way of example. In this case, first, the radio base station designates a preamble signal that the radio base station permits the mobile station to use. The mobile station transmits the designated preamble signal via the random access channel. The radio base station measures a timing error on the basis of the random access signal received from the mobile station (the preamble signal designated by the radio base station), and instructs the mobile station to adjust the timing. Following the instruction, the mobile station adjusts the transmission timing. Subsequently, the radio base station transmits downlink data to the mobile station. The mobile station then transmits ACK (ACKnowledgement)/NACK (Negative ACKnowledgement), which is indicative of the result of the data reception, to the radio base station at the corrected timing. Thus, at the time of downlink data communication, the random access channel can be used for synchronizing the uplink timing.
Let us now consider the case where the mobile station transmits uplink data to the radio base station. In this case, first, the mobile station selects a preamble signal from among multiple candidates, with the use of random numbers, and transmits the selected preamble signal via the random access channel. On receiving the random access signal (preamble signal not designated by the radio base station) from the mobile station, the radio base station allocates, to the mobile station, an uplink radio resource for transmitting control data. Using the allocated uplink radio resource, the mobile station transmits a data send request (e.g., BSR: Buffer Status Report). The radio base station allocates an uplink radio resource with a size matching the data send request received from the mobile station. Subsequently, the mobile station transmits the uplink data to the radio base station. In this manner, the mobile station can initiate uplink data transmission by using the random access channel.
The random access method described above is, however, associated with the problem that the procedure is executed independently with respect to every cause of the occurrence of a random access, which makes the processing inefficient. Specifically, in the case of a random access at the time of downlink data communication, the allocation of an uplink radio resource is judged to be unnecessary, because only the uplink timing has to be synchronized. Consequently, where the mobile station needs to transmit control data during the downlink data communication (e.g., where the mobile station initiates uplink data communication), the random access procedure must be executed separately for the allocation of an uplink radio resource.